Solid-gel precursor solutions and methods for the fabrication of polymetallicsiloxane coating films

ABSTRACT

Solutions and preparation methods necessary for the fabrication of metal oxide cross-linked polysiloxane coating films are disclosed. The films are useful in provide heat resistance against oxidation, wear resistance, thermal insulation, and corrosion resistance of substrates. The sol-gel precursor solution comprises a mixture of a monomeric organoalkoxysilane, a metal alkoxide M(OR) n  (wherein M is Ti, Zr, Ge or Al; R is CH 3 , C 2  H 5  or C 3  H 7  ; and n is 3 or 4), methanol, water, HCl and NaOH. The invention provides a sol-gel solution, and a method of use thereof, which can be applied and processed at low temperatures (i.e., &lt;1000° C.). The substrate can be coated by immersing it in the above mentioned solution at ambient temperature. The substrate is then withdrawn from the solution. Next, the coated substrate is heated for a time sufficient and at a temperature sufficient to yield a solid coating. The coated substrate is then heated for a time sufficient, and temperature sufficient to produce a polymetallicsiloxane coating.

This invention was made with Government support under contract numberDE-AC02-76CH00016, between the U.S. Department of Energy and AssociatedUniversities, Inc. The Government has certain rights in the invention.

This is a division of co-pending application Ser. No. 590,770 filed Oct.1, 1990, now U.S. Pat. No. 5,110,863.

BACKGROUND OF THE INVENTION

The present invention includes solutions and the preparation methodsnecessary for the fabrication of metal oxide cross-linked polysiloxanecoating films which are useful in providing heat resistance againstoxidation, wear resistance, thermal insulation, and corrosion resistanceof substrates.

In the past, ceramic coatings on metallic and plastic substrates havenot been widely used primarily because many ceramic coatings can beapplied and processed only at high temperatures (i.e., only attemperatures above 1000° C.) using expensive and time-consuming methodssuch as chemical vapor deposition. Therefore, aluminum alloys, plasticsand other materials with low melting points were not easily protected.

U.S. Pat. No. 4,584,280 to Nanao discloses a process for preparing aporous ceramic film by applying an anhydrous solution containing anorganometallic compound and a multifunctional organic compound to asubstrate and thermally decomposing the substrate. The organometalliccompound may be titanium alkoxide. Examples of the multifunctionalorganic compound include such organic compounds as glycerine,1,4-butenediol, pentaerythritol, dextrin, arginic acid, methylcellulose, ethyl cellulose, hydroxyethyl cellulose, carboxymethylcellulose, carboxymethyl starch, hydroxyethyl starch, polyvinylalcohol,and mixtures thereof. The thermal decomposition is conducted at atemperature of not less than 200° C., and then, if necessary, the coatedsubstrate is baked. Nanao does not teach the formation of apolymetallicsiloxane film at low temperatures as does the presentinvention.

U.S. Pat. Nos. 4,455,414 and 4,347,347, to Yajima, et al. disclose anorganic copolymer composed of a polycarbosilane portion and apolymetallicsiloxane portion cross-linked with each other and theprocess of making it. Neither patent teaches the formation of apolymetallicsiloxane film at low temperature as does the presentinvention.

U.S. Pat. No. 4,028,085 to Thomas discloses the combination of ahydrolyzable metal alkoxide with a partially hydrolyzed silicontetraalkoxide to form a metallicsiloxane solution. Thomas does not teachthe application of the solution to a substrate nor does it teach theheating of the solution to create a ceramic-type polymetallicsiloxanecoating.

It is, therefore, an object of the present invention to provide apolymetallicsiloxane sol-gel precursor solution which can be used in thepreparation of metal oxide cross-linked polysiloxane coating films whichare useful in providing heat resistance against oxidation, wearresistance, thermal insulation, and corrosion resistance of substrates.

It is an object of the invention to provide a sol-gel solution, and amethod of use thereof, which can be applied and processed at lowtemperatures (i.e., at temperatures of less than 1000° C.).

A further object of the invention is to provide a polymetallicsiloxanesol-gel precursor solution which will adhere well and have anappropriate expansion coefficient, especially during temperaturecycling, so that separation of the coating film from the substrate willnot occur.

BRIEF DESCRIPTION OF THE INVENTION

This invention relates to the formulation of sol-gel precursor solutionsand the preparation methods necessary for the fabrication of the metaloxide cross-linked polysiloxane coating films. The metal oxidecross-linked polysiloxane coating film enhances heat resistance againstoxidation, wear resistance, thermal insulation and corrosion resistanceof substrates such as aluminum, steel, magnesium, and titanium. Thesol-gel precursor solution includes a mixture of a monomericorganoalkoxysilane, a metal alkoxide M(OR)_(n) (wherein M is atransition metal; R is CH₃, C₂ H₅ or C₃ H_(7;) and n is 3 or 4),alcohol, water and a chlorine containing acid. Suitably M can includeTi, Zr, Ge and Al. Preferably the alcohol is methanol, ethanol orpropanol. The invention provides a sol-gel solution, which can beapplied and processed at low temperatures (i.e., <1000° C.). Preferably,NaOH is used to adjust the pH of the solution to about 7.5.

Preferably, the monomeric organoalkoxysilane is selected from the groupconsisting of N[3-(triethoxysilyl) propyl]imidazole (TSPI) andN[3-triethoxysilyl)propyl]-4,5-dihydroimidazole (TSPDI). In a preferredembodiment the amount of HCl is sufficient to provide a clear solutionand acts as a hydrolysis accelerator. In another preferred embodimentthe ratio of imidazole containing monomeric organoalkoxysilane to metalalkoxide is in the range of about 80/20 to about 50/50 by weight (i.e.,the solution comprises 18-35 wt % TSPI or TSPDI, 9-18 wt % Ti(OC₂ H₅)4,21-26 wt % methanol, 13-29 wt % HCl and 14-17 wt % water). The sol-gelsolution is miscible with water and the thickness of the coating filmscan be adjusted by adding an appropriate amount of water to thesolution.

The substrate can be coated by immersing it in the above mentionedsolution at ambient temperature. The substrate is then withdrawn fromthe solution. Next, the coated substrate is heated for a time sufficientand at a temperature sufficient to yield a solid coating. The coatedsubstrate is then heated for a time sufficient, and at a temperaturesufficient, to produce a polymetallicsiloxane coating.

To date, ceramic coatings on metallic and plastic substrates have notbeen widely employed for several reasons. First, coatings must adherewell and have an appropriate expansion coefficient. This is especiallytrue during temperature cycling, otherwise, separation of the coatingfilm from the substrate will occur. Second, many ceramic coatings can beapplied and processed as coatings only at high temperatures(i.e., >1000° C.) using expensive and time-consuming methods such aschemical vapor deposition. Therefore, the instant sol-gel solution, andthe preparation methods for the formulation of metal oxide cross-linkedpolysiloxane coatings films, are advantageous in that they permit theapplication of an effective polymetallicsiloxane coating at atemperature which more easily permits the use of aluminum alloys andother low melting point materials.

For a better understanding of the invention, together with other andfurther objects, reference is made to the following description, and itsscope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the IR absorption spectra for powder samples ofvarious GPS/Ti(OC₂ H₅)₄ ratios heat treated at 300° C.: a) 100/0; b)80/20; c) 60/40; d) 40/60 and e) 30/70.

FIG. 2 details the IR absorption spectra for powder samples of variousGPS/Ti(OC₂ H₅)₄ ratios heated at 400° C.: a) 100/0; b) 80/20; c) 60/40;d) 40/60; and 3) 30/70.

FIG. 3 details the IR absorption spectra for powder samples of variousGPS/Ti(OC₂ H₅)₄ ratios heated at 500° C.: a) 100/0; b) 80/20; c) 60/40;d) 40/60; and 3) 30/70.

FIGS. 4a and 4b illustrate surface morphologies for GPS/Ti(OC₂ H₅)₄coatings treated at 300° C. The micrographs correspond to the followingGPS/TI(OC₂ H₅)₄ ratios; FIG. 4a--100/0 and FIG. 4b--60/40.

FIG. 5 graphically illustrates the variation in corrosion current(I_(corr)) for aluminum substrates coated with various GPS/Ti(OC₂ H₅)₄ratio systems as a function of the film-treatment temperature.

FIG. 6 illustrates the changes in IR absorbance corresponding to theSi--O--Ti bond at approximately 930 m⁻¹ for Ti compound-incorporatedorganosilanes preheated at temperatures within the range of 200° to 500°C.

FIGS. 7a and 7b detail SEM images for TSPI 7a and APS 7b coating filmsheated at 200° C.

FIG. 8a and 8b details the surface morphologies for TSPDI 8a and APS 8bsystem coatings heat treated at 300° C.

FIG. 9 illustrates the SEM micrograph for the TSPDI coating system heattreated at 300° C.

FIG. 10 illustrates the IR spectra for 350° C.-annealed TSPDI/M(OC₃ H₇)₃or 4 for the (a) 100% TSPDI, (b) TSPDI/Zr(OC₃ H₇)₄ (50:50 ratio) and (c)TSPDI/Ti(OC₃ H₇)₄ (50:50 ratio) systems.

FIG. 11 illustrates the polyzirconicsiloxane (PZS) film derived from the70/30 TSPDI/Zr(OC₃ H₇)₄ sol-gel solution.

FIGS. 12a and 12b illustrate surface features of pyrolysis-induced PTScoating films; FIG. 12a illustrates a 70/30 TSPI/Ti(OC₃ H₇)₄ ratiosystem and FIG. 12b illustrates a 50/50 TSPI/Ti(OC₃ H₇)₄ ratio systems.

DETAILED DESCRIPTION OF THE INVENTION

The sol-gel precursor solution of the present invention includes amixture of a monomeric organoalkoxysilane, a metal alkoxide M(OR)n(wherein M is a suitable transition metal); R is CH₃, C₂ H₅ or C₃ H₇ ;and n is 3 or 4), alcohol (such as methanol, ethanol or propanol),water, and a chlorine containing acid (such as HCl). Suitably, M may beTi, Zr, Ge or Al. Preferably, the pH of the solution is adjusted toabout 7.5 (for reasons of handling safety) by the addition of NaOH.Among the monomeric organoalkoxysilanes which can be used with thepresent invention are those listed in Table 1. In a preferredembodiment, the monomeric organoalkoxysilane contains an imidazolegroup, for example, N[3-(triethoxysilyl) propyl]-4,5 imidazole (TSPI)and N[3-triethoxysilyl)propyl]-4,5-dihydroimidazole (TSPDI).

                  TABLE 1                                                         ______________________________________                                        Organosilane/Chemical Formula                                                 ______________________________________                                        3-glycidoxypropyltrimethoxysilane (GPS)                                        ##STR1##                                                                     3-aminopropyltrimethoxysilane (APS)                                           H.sub.2 N(CH.sub.2).sub.3Si(OCH.sub.3).sub.3                                  N-[3-(triethoxysilyl)propyl]imidazole (TSPI)                                   ##STR2##                                                                     N-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole (TSPDI)                      ##STR3##                                                                     ______________________________________                                    

The film-forming precursor solution can be prepared by incorporating anorganoalkoxysilane/metal alkoxide (M(OR)n wherein M is a suitabletransition metal such as Ti, Zr, Ge or Al; R is CH₃, C₂ H₅ or C₃ H₇ ;and n is 3 or 4) into an alcohol/water mixing medium containing anappropriate amount of an acid containing chlorine. Suitably, the alcoholmay be methanol, ethanol or propanol. Preferably, the acid is HCl. Theacid acts as a hydrolysis accelerator and produces a clear precursorsolution. The addition of the acid aids in the formation of a uniformcoating film on the metal substrate. When the precursor solution is usedas a coating material for a metal substrate, the pH of the solution ispreferably adjusted to approximately 7.5 by the addition of anappropriate amount of a suitable base such as, for example KOH or NaOH.Prior to addition of the base, the solution will be very acidic (i.e.,it will have a pH of from about 1.0 to about 3.5). The base makes thesolution safer to handle.

The aluminum substrate used in the following examples was 2024-T3 cladaluminum sheet containing the following chemical constituents: 92 wt. %Al, 0.5 wt. % Si, 0.5 wt. % Fe, 4.5 wt. % Cu, 0.5 wt. % Mn, 1.5 wt. %Mg, 0.1 wt. % Cr, 0.25 wt. % Zn and 0.15 wt. % other elements.

The oxide etching of the aluminum was carried out in accordance with awell known commercial sequence called the Forest Products Laboratory(FPL) process. As the first step in the preparation, the surfaces werecleansed with acetone to remove any organic contamination. They werethen immersed in chromic-sulfuric acid (Na₂ Cr₂ O₇.2H₂ O: H₂ SO₄:Water=4:23:73 by weight) for 10 min at 80° C. After etching, the freshoxide surfaces were washed with deionized water at 30° C. for 5 min, andsubsequently dried for 15 min at 50° C.

The substrate can be coated by immersing it in the above mentionedsolution at ambient temperature. The substrate is then withdrawn fromthe solution. Next, the coated substrate is heated for a time sufficientand at a temperature sufficient to yield a solid coating. The coatedsubstrate is then heated for a time sufficient, and at a temperaturesufficient, to produce a polymetallicsiloxane coating.

A thinner polymetallicsiloxane coating may be obtained by diluting thesol-gel precursor solution with water.

Ti(OC₂ H₅)₄ -Modified GPS System

Coating of the aluminum surfaces using the sol-gel system was performedin accordance with the following sequence. First, the FPL-etchedaluminum substrate was immersed in the precursor solution at ambienttemperature. The substrate was then withdrawn slowly from the soakingbath, after which the substrate was heated for 20 hrs. at a temperatureof 100° C. to yield a solid coating. The samples were subsequentlyheated for 20 min. at temperatures ranging from 200° to 500° C.

The mix compositions for the GPS/Ti(OC₂ H₅)₄ based precursor solutionsystems are given in Table 2. For each formulation, the GPS to Ti(OC₂H₅)₄ ratio was varied so that the concentration of HCl needed to producea clear precursor solution was dependent mainly on the GPS/Ti(OC₂ H₅)₄ratio. As the proportion of Ti(OC₂ H₅)₄ increased, the required amountof HCl was increased to form Ti compounds which were susceptible tohydrolysis. The HCl-catalyzed hydrolysis of Ti(OC₂ H₅)₄ is as follows:

    .tbd.Ti--OC.sub.2 H.sub.5 +H.sup.+ +Cl.sup.- →.tbd.Ti--Cl+C.sub.2 H.sub.5 OH .tbd.Ti--Cl+H.sub.2 O→.tbd.Ti--OH+HCl

                                      TABLE 2                                     __________________________________________________________________________    Compositions of clear precursor solutions used for                            various GPS/Ti(OC.sub.2 H.sub.5).sub.4 ratios                                 GPS/Ti(OC.sub.2 H.sub.5).sub.4                                                         GPS  Ti(OC.sub.2 H.sub.5).sub.4                                                          CH.sub.3 OH                                                                        Water                                                                              HCl (wt. %)/                                    (wt. ratio)                                                                            (wt. %)                                                                            (wt. %)                                                                             (wt. %)                                                                            (wt. %)                                                                            [GPS + Ti(OC.sub.2 H.sub.5).sub.4               __________________________________________________________________________    100/0    50    0    30   20   10                                              80/20    40   10    30   20   10                                              60/40    30   20    30   20   20                                              40/60    20   30    30   20   30                                              __________________________________________________________________________

The hydroxylated titania derived from the hydrolysis of Ti(OC₂ H₅)₄appears to react preferentially with the C--Cl groups, rather than thesilanol groups (Si--OH). The silanol groups are formed by hydrolysis ofthe methoxysilane groups in GPS. A possible condensation reactionoccurring between the C--Cl in the polymeric organosilanes and thehydroxy groups in the hydrated Ti compounds is shown below: ##STR4##

Upon heating to 300° C., a large number of carbon containing groups suchas CH₂ O and CH₃ CHO are eliminated from the Ti incorporatedorganopolysilane networks. This can be seen in the following equation:##STR5## The conversion of the Ti incorporated organopolysilane networksinto the polymetallicsiloxane network structure occurs at about 300° C.

At temperatures of about 300° and above, pyrolytic changes inconformation appear to occur. The pyrolytic changes result in theelimination of numerous organic groups from the Ti-incorporatedorganopolysilane network structures. Once the transition is completed,the Ti elements located in the networks act as a crosslinking agentwhich connect directly between the polysiloxane chains. The extent of Ticrosslinking depends mainly on the GPS/Ti(OC₂ H₅)₄ ratio. Samples for IRanalysis were prepared by incorporating the powdered samples into KBrpellets. The presence of Si--O--Ti linkages in the PTS is indicated byan IR absorption peak at approximately 930 cm⁻¹. The absorptionintensity around 930 cm⁻¹ becomes weaker as the proportion of Ti(OC₂H₅)₄ is increased. This is illustrated in FIG. 1. The presence of thebonds at 930 cm⁻¹ illustrates the formation of a polytitanosiloxane filmat a low temperature (i.e., less than 1000° C.).

When the heat treatment temperature was increased to 400° C., the peakin the 2900 cm⁻¹ region of the IR spectra for all of the GPS samplesdisappeared. This is shown in FIG. 2 and suggests that all the residualorganic compounds were nearly removed from the PTS networks. Thespectral features for the 400° C.-treated samples were similar to thosefor the 300° C.-treated ones with the exception of the disappearance ofthe 2900 cm⁻¹ scale from 400° C. treated sample. Comparing the resultsat 500° C. (see FIG. 3), with those at 400° C. (FIG. 2), no specificchanges or shifts in the absorption bands for any samples can be seen.

When the film treatment temperature was raised to 300° C., samplescontaining a GPS/Ti(OC₂ H₅)₄ ratio of 100/0 experienced severe damage.This is shown in FIG. 4a. The failure appears to be due to pyrolyticchanges in conformation of the polymeric organosilane. These pyrolyticchanges result from the elimination of organic species from the networkstructure and result in excessive shrinkage of the film. The SEM(scanning electron microscope) microstructure view of the 60/40 ratiofilm (FIG. 4b) disclosed a much lower magnitude in shrinkage and/orstress cracks. This strongly suggests that the cross-linking ability ofthe Ti compounds, which connect directly between the polysilane chains,acts significantly to suppress the development of stress cracks. It istheorized that the network structure of the PTS polymers, formed bypyrolytically induced conformational changes in Ti compound modifiedorganosilane polymers, contributes to the maintenance of film shape athigh temperatures. The amount of cracking can be reduced by diluting thesol gel precursor solution with water. The dilution of the sol gelprecursor solution results in the formation of a thinnerpolymetallicsiloxane coating.

Corrosion protection data for the above-coated substrates were obtainedfrom the polarization curves for PMS coated FPL etched aluminum samplesupon exposure to an aerated 0.5M sodium chloride solution at 25° C. Thetypical cathodic-anodic polarization curves of log current density vs.potential for the coated samples were similar to those reported byseveral investigators for other materials (G. A. Dibari and H. J. Read,Corrosion, 27 (1971) 483; Z. A. Foroulis and M. J. Thubriker,Electrochim. Acta, 21 (1976) 225; and A. V. Pocius, in K. L. Mittal(ed.), Adhesion Aspects of Polymeric Coatings, Plenum Press, New York,1983, pp. 173-192).

The corrosion protective performance of the coatings was evaluated by anelectrochemical procedure involving measurement of the corrosioncurrent, I_(corr), by extrapolation of the cathodic Tafel slope. Thevariation in the I_(corr) value was plotted as a function of thetreatment temperature. These results are depicted in FIG. 5. As seen inFIG. 5, the protective ability of the coatings depends primarily on theGPS/Ti(OC₂ H₅)₄ ratio and the treatment temperature. A low I_(corr)value indicates good corrosion protection.

The I_(corr) -temperature relations for the 80/20 and 60/40 ratiocoatings indicate that although microcracks form on the film surface attemperatures ≧300° C., the I_(corr) values after treatment at 400° C.are almost equal to those for the coatings pretreated at 100° C. Thissuggests that PTS coating films at 100° C. formed from in-situconformation changes at 400° C. provided corrosion protection foraluminum.

Ti(OC₂ H₅)₄ --Modified Organosilanes

Coating of the aluminum surfaces using the sol-gel system was performedin accordance with the following sequence. First, the FPL-etchedaluminum substrate was immersed in the precursor solution at ambienttemperature. The substrate was then withdrawn slowly and heated for 20hr at a temperature of 100° C. to yield a solid coating. The sampleswere subsequently heated for 20 min at temperatures ranging from 200° to500° C.

A film-forming precursor solution composed of 30 wt % of the particularorganosilane, 20 wt % Ti(OC₂ H₅)₄, 30 wt % CH₃ OH and 20 wt % water wasemployed to produce the PTS polymers. The required concentrations of theHCl hydrolysis promoter needed to prepare clear precursor solutions weredependent upon the species of organosilane, and for the TSPDI system was30% by weight of total mass of organosilanes and Ti(OC₂ H₅)₄.

The presence of Si--O--Ti linkages in the PTS can be readily identifiedfrom the IR absorption peak at approximately 930 cm⁻¹. The extent of thedensification of the Si--O--Ti linkages was estimated by comparing theabsorbencies at approximately 930 cm⁻¹ for the PTS samples derived fromthe various organosilane-Ti(OC₂ H₅)₄ systems. As previously discussed,samples for the IR analysis were prepared by incorporating the powderedsamples into KBr pellets. FIG. 6 summarizes the resulting variations inabsorbance plotted as a function of treatment temperature. The dataindicates that the extent of densification of Si--O--Ti bonds isdependent upon the reactive organic functional groups attached to theterminal carbon of the methylene chains within the monomericorganosilane structures.

An absorption peak at approximately 930 cm⁻¹ was not detected for the200° C.-treated GPS- and TSPDI-Ti(OC₂ H₅)₄ systems. This indicates thata PTS containing a highly densified Si--O--Ti bond was not formed atthis temperature. A prominent IR peak at approximately 930 cm⁻¹ wasobserved for the GPS and TSPDI-Ti(OC₂ H₅)₄ systems when the samples wereheated at 300° C. for 20 min. An absorption peak at approximately 930cm⁻¹ was observed for the 200° C. treated APS-Ti(OC₂ H₅)₄ system. Thisindicates that PTS, containing a highly densified Si--O--Ti bond, wasformed at these temperatures. This illustrates the formation of apolymetallicsiloxane coating at a low temperature (i.e., less than 1000°C.). Beyond this temperature, the absorbance value increased slowly,suggesting that the in-situ conversion of the Ti compound-incorporatedorganosilane polymers into PTS progressively occurs at temperaturesranging from about 200 to about 300° C.

FIG. 7 illustrates the SEM images obtained for coating film surfacespreheated at 200° C. Except for the development of few microcracks, theAPS and TSPDI coatings [FIG. 7a and b] exhibit excellent surfaces.

The SEM micrographs of these coating systems after being exposed to airfor 20 min at 300° C. are shown in FIG. 8. The APS and TSPDI coatings(FIG. 8a and b) showed no film damage with the exception of theappearance of a clear crack line.

Heat damage and distortion of the aluminum substrate was apparent, butafter heating for 20 min at 500° C., the TSPDI coating was not damaged[See FIG. 9a]. Accordingly, PTS coating films derived from the Ti(OC₂H₅)₄ -TSPDI system appear to have the most stable Si--O--Ti bonds in thePTS network structure. This may be due to moderate densification of theSi--O--Ti bonds in the PTS network structure.

The corrosion protective performance of PTS coatings derived fromvarious organosilane-Ti(OC₂ H₅)₄ systems was determined by comparing thecorrosion current, (I_(corr)) values determined from the cathodic Tafelslopes of the various organosilane-Ti(OC₂ H₅)₄ systems. The corrosiontests in this study were performed on PTS coatings formed on theFPL-etched aluminum at 300°, 400°, and 500° C. The resultant changes inI_(corr), for these coating specimens are summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                                  I.sub.coor value (μA)                                                      obtained after pretreatment at                                      Coating system                                                                            300° C.                                                                           400° C.                                                                           500° C.                              ______________________________________                                        GPS-Ti(OC.sub.2 H.sub.5).sub.4                                                            3.5 × 10.sup.-1                                                                    6.0 × 10.sup.-1                                                                    0.5                                         APS-Ti(OC.sub.2 H.sub.5).sub.4                                                            8.5 × 10.sup.-2                                                                    5.8 × 10.sup.-1                                                                    1.2                                         TSPDI-Ti(OC.sub.2 H.sub.5).sub.4                                                          2.0 × 10.sup.-2                                                                    4.6 × 10.sup.-1                                                                    9.8 × 10.sup.-1                       TSPI-Ti(OC.sub.2 H.sub.5).sub.4                                                           2.5 × 10.sup.-2                                                                    4.9 × 10.sup.-1                                                                    9.9 × 10.sup.-1                       ______________________________________                                    

After treatment at 300° C., the lowest I_(corr) value of 2.0×10⁻² μA wasmeasured on the PTS coatings derived from the TSPDI system. The APSsystem produced the next lowest I_(corr) value. These values wereapproximately two orders of magnitude less than that for the TS system.The data indicates that the I_(corr) values for all of the PTS coatingsformed at ≧300° C. increased as the film treatment temperature wasraised. This is probably due to the increased size and number of cracksin the films. PTS coatings derived from the TSPDI system imparted thebest corrosion protection, and at 500° C., the I_(corr) value was stillon the order of 10⁻¹ μA.

Ti(OC₃ H₇)₄, Zn(OC₃ H₇)₄ and Al(OC₃ H₇)₃ --Modified Organosilanes

The mix compositions for the Ti(OC₃ H₇)₄, Zn(OC₃ H₇)₄ and Al(OC₃ H₇)₃sol-gel precursor solutions are listed in Table 4. In order to produce aclear precursor solution it was very important to add a chlorinecontaining acid such as HCl. The chlorine containing acid acted as ahydrolysis accelerator and aided in the formation of a uniform coatingfilm on the metal substrate.

                                      TABLE 4                                     __________________________________________________________________________    Compositions of Clear Precursor Solutions Used in Various M(OC.sub.3          H.sub.7).sub.n * - Modified TSPI Systems.                                                                                 HCl,                              TSPI/M(OC.sub.3 H.sub.7).sub.n *                                                        TSPDI                                                                             Zr(OC.sub.3 H.sub.7).sub.4,                                                          Ti(OC.sub.3 H.sub.7).sub.4,                                                          Al(OC.sub.3 H.sub.7).sub.3,                                                          CH.sub.3 OH,                                                                       Water,                                                                            wt %/TSPI +                       wt ratio  wt %                                                                              wt %   wt %   wt %   wt % wt %                                                                              M(OC.sub.3 H.sub.7).sub.4or3      __________________________________________________________________________    100/0     50  --     --     --     30   20  12                                70/30     35  15     --     --     30   20  20                                50/50     25  25     --     --     30   20  30                                70/30     35  --     15     --     30   20  15                                50/50     25  --     25     --     30   20  25                                70/30     35  --     --     15     30   20  40                                50/50     25  --     --     25     30   20  50                                __________________________________________________________________________     *M: Zr, Ti and Al                                                             n: 3 or 4                                                                

The substrates were coated by immersing an FPL-etched aluminum substrateinto the precursor solution at ambient temperature. The substrate wasthen withdrawn from the precursor solution. Next, the substrate was heattreated at 150° C. for 20 hrs. The 150° C. heat treatment results in theremoval of water and methanol from the precursor solution coating andproduces a sintered coating. The substrates coated with the Ti(OC₃ H₇)₄and Zn(OC₃ H₇)₄ sol-gel precursor solutions were heated for 30 minutesat 350° C. to form polyzirconicsiloxane and polytitanosiloxane coatings.The substrates coated with the Al(OC₃ H₇)₃ sol-gel precursor solutionswere heated for 30 minutes at 200° C. to form a polyaluminosiloxanecoating.

The HCI catalyzed hydrolysis-polycondensation reaction occurred in thefollowing manner: ##STR6## It is believed that the hydroxyl groupsderived from the HCl-catalyzed hydrolysis of Zr(OC₃ H₇)₄ and Ti(OC₃H₇)₄, react preferentially with the Cl in Cl-substituted end groups inthe silane compound, rather than the silanol groups which are formed byhydrolysis of the ethoxysilyl groups in the TSPDI. The proposed reactionmechanism for this is shown below: ##STR7##

It is believed that the reaction of the halide with the OH in thehydroxylated metals favors the elimination of hydrogen chloride. Theformation of Cl-terminated end groups plays an important role increating the M--O--C linkages.

The reaction process for the Al(OC₃ H₇)₃ /TSPDI system is different thanthose of the Ti(OC₃ H₇)₄ /TSPDI and Ti(OC₃ H₇)₄ /TSPDI systems. Apolymeric organoaluminosilane network is formed when the Al(OC₃ H₇)₃/TSPDI system is heated to 150° C. and is believed to have the followingstructure: ##STR8##

IR studies were performed on the Ti(OC₃ H₇)₄ /TSPDI and Zr(OC₃ H₇)₄/TSPDI samples after they had been heated for 30 minutes in air at 350°C. The samples had previously been heated at 150° C. for 20 hours. TheIR analyses were conducted using the KBr method which incorporates thepowder samples of 2 to 3 mg into KBr pellets of approximately 200 mg.FIG. 10 illustrates the IR spectra for a) 100% TSPDI, b) TSPDI/Zr(OC₃H₇)₄ (in a 50:50 ratio) and c) TSPDI/Ti(OC₃ H₇)₄ (in a 50:50 ratio)samples heat treated at 150° C. for 20 hours and 350° C. for 30 minutes.The presence of a polymetallicsiloxane is indicated by an IR peak withinthe area of about 910 cm⁻¹ to about 960cm⁻¹.

The TSPDI/Zr(OC₃ H₇)₄ system (FIG. 10(b)) had an IR peak at 950 cm⁻¹.The TSPDI/Ti(OC₃ H₇)₄ system (FIG. 10c) had an IR peak at 930 cm⁻¹.These peaks signify the formation of polymetallicsiloxane, at a lowtemperature (i.e., less than 1000° C.), by the process shown below:##STR9##

The 350° C. heating results in the elimination of numerous organicgroups permitting the Zr and Ti metal oxides to act as crosslinkingagents which connect the polysiloxane chains to formpolyzirconicsiloxane and polytitanosiloxane.

Characteristics of PMS (Coating Films Derived From TSPDI/Ti(OC₃ H₇)₄,TSPDI/Zr(OC₃ H₇)₄ and TSPDI/Al(OC₃ H₇)₃ Precursor Systems

Thin coating films were obtained by diluting 20 g of the precursorsolutions listed in Table 4 with 80 g of deionized water. The FPL-etchedaluminum substrate was immersed into the diluted precursor solution. Thesubstrate was withdrawn from the solution and heated for 20 hours at150° C. The sintered samples were then heated at 350° C. for 30 minutesto form the pyrolysis induced PMS coating films. The thickness of thePMS film deposited on the substrate was determined using a surfaceprofile measuring system. The average thickness of the films derivedfrom the precursor solution consisting of 100/0, 70/30, and 50/50TSPDI/M(OC₃ H₇)₃ or 4 ratios, ranged from approximately 0.2 toapproximately 0.4 μm.

FIG. 11 illustrates the polyzirconicsiloxane (PZS) film derived from the70/30 TSPDI/Zr(OC₃ H₇)₄. This PZS film had relatively few microcracks.The amount of cracking can be reduced by diluting the sol gel precursorsolution with water. The dilution of the sol gel precursor solutionresults in the formation of a thinner polymetallicsiloxane coating.

Ideally, a PMS coating surface will have a uniform film free of cracksand pits. These characteristics were observed in the 50/50 TSPDI/Ti(OC₃H₇)₄ ratio derived polytitanosiloxane (PTS) film illustrated in FIG.12a. FIG. 12b illustrates the 70/30 TSPDI/Ti(OC₃ H₇)₄ ratio derived PTSfilm. The 70/30 ratio film has a few microcracks. A thinnerpolymetallicsiloxane coating may be produced by diluting the sol gelprecursor solution with water.

Corrosion protection data for the polytitanosiloxane andpolyzirconicsiloxane coated substrates were obtained from thepolarization curves for PMS coated FPL etched aluminum samples uponexposure to an aerated 0.5M sodium chloride solution at 25° C. Toevaluate the protective performance of the coatings, the corrosionpotential (E_(Corr)) and corrosion current (I_(Corr)) were determinedfor the polarization curves. E_(Corr) is defined as the potential at thetransition point from cathodic to anodic polarization curves. I_(Corr)values were measured by extrapolation of the cathodic Tafel slope. Theseresults are summarized in Table 5.

                  TABLE 5                                                         ______________________________________                                        Corrosion Potential, E.sub.corr and Corrosion Current, I.sub.corr, Values     for                                                                           PMS-Coated and Uncoated Aluminum Specimens.                                   Coating Systems,          I.sub.corr,                                         (TSPI/M(OC.sub.3 H.sub.7).sub.4or3)                                                             E.sub.coor *                                                                          μA                                               ______________________________________                                        Uncoated (blank)  -0.725  2.5                                                 PS (100/0)        -0.695  1.8                                                 PZS (70/30)       -0.625  7.8 × 10.sup.-1                               PZS (50/50)       -0.710  1.5                                                 PTS (70/30)       -0.589  1.8 × 10.sup.-1                               PTS (50/50)       -0.596  1.6 × 10.sup.-1                               ______________________________________                                    

As seen, the major effect of these PMS coatings on the corrosionprotection of aluminum is to move the E_(Corr) value to less negativepotentials and to reduce the cathodic current (I_(corr)).

The samples coated with PTS produced significantly higher E_(Corr)values, and significantly lower I_(Corr) values, than the uncoatedsamples. This strongly suggests that the PTS coating films will serve toprovide good corrosion resistance from a sodium chloride solution andwill minimize the corrosion rate of the aluminum.

Thus, while there have been described what are the presentlycontemplated preferred embodiments of the present invention, thoseskilled in the art will realize that changes and modifications may bemade thereto without departing from the scope of the invention, and itis intended to claim all such changes and modifications as fall withinthe true scope of the invention.

I claim:
 1. A method of preparing a polymetallicsiloxane coating on asubstrate comprising:(a) coating the substrate with a solutioncomprising a monomeric organoalkoxysilane, containing an imidazolegroup, a metal alkoxide and a chlorine containing acid in analcohol/water medium; (b) heating the coated substrate for a timesufficient and at a temperature sufficient to yield a solid coating; and(c) further heating said substrate for a time sufficient and at atemperature sufficient to produce a polymetallisiloxane coating uponsaid substrate.
 2. The method of claim 1 wherein the alcohol is selectedfrom the group consisting of methanol, ethanol and propanol.
 3. Themethod of claim 1, wherein the acid is HCl.
 4. The method of claim 1wherein the metal alkoxide is of the formula M(OR)_(n), wherein M is asuitable transition metal; R is CH₃, C₂ H₅ or C₃ H₇, and, n is 3 or 4.5. The method of claim 4 wherein M is Ti, Ge, Zr or Al.
 6. The method ofclaim 1 further comprising adjusting the solution to have a pH of about7.5.
 7. The method of claim 6 wherein said pH is adjusted by adding NaOHor KOH.
 8. The method of claim 1 wherein said step (b) is performed at atemperature of about 100° C. to about 150° C. for a time about 20 hours.9. The method of claim 1 wherein said step (c) is performed at atemperature of about 200° C. to about 350° C. for a period of about20-30 minutes.
 10. The method of claim 1 wherein the monomericorganoalkoxy silane is selected from the group consisting ofN(3-(triethoxysilyl) propyl) imidazole and N(3-(triethoxysilyl)propyl)-4,5-dihydroimidazole.
 11. The method of claim 4 wherein themetal alkoxide is selected from the group consisting of Ti(OC₂ H₅)₄,Ti(OC₃ H₂)₄, Zr(OC₃ H₇)₄ and Al(OC₃ H₇)₃.
 12. The method of claim 2wherein the alcohol is methanol.
 13. The method of claim 1 wherein thesolution used in step (a) has a ratio of monomeric organoalkoxysilane tometal alkoxide in the range of about 80/20 to about 50/50 by weight. 14.The method of claim 1 wherein the solution has the following weightpercentages of substituents, 18-35 wt %N[3-(triethoxysily)propy]imidazole 9-18 wt % Ti(OC₂ H₅)₄, 21-26 wt %methanol, 13-29 wt % HCl and 14-17 wt % water.
 15. The method of claim 1wherein the solution has the following weight percentages ofsubstituents 18-35 wt % N[3-triethoxysilyl)propyl]4,5dihydroimidazolemonomer, 9-18 wt % Ti(OC₂ H₅)₄, 21-26 wt % methanol, 13-29 wt % HCl and14-17 wt % water.